This invention relates to a quadrature surface coil system for detecting magnetic fields induced in a body being examined in a magnetic resonant imaging system.
Quadrature surface coil systems for use in magnetic resonance imaging apparatuses are known. For example, reference is made to U.S. Pat. No. 4,816,765, incorporated by reference herein which discloses a quadrature coil system which is used in a magnetic resonance imaging apparatus. The detection coil is one referred to as a surface coil in that it is placed adjacent a surface of a body being examined by the magnetic resonance imaging system apparatus. As disclosed in the aforementioned patent, the detection coil comprises a stack of two coils with locally mutually perpendicular related B fields. A first coil is formed as a butterfly coil having a B field extending parallel to the plane of the coil and the second coil is formed of a single flat coil having a B field extending transversely the plane of the coil. The coils are arranged in a fixed mutual orientation so that for each of the coils reciprocally applied fluxes are compensated for.
The present invention is an improvement of that quadrature coil system. Such quadrature coil systems as disclosed in the aforementioned U.S. patent have the disadvantage in that the coils must be typically arranged planar. The reason for the planar arrangement is that should the coils be bent or otherwise contoured, typically to the shape of the surface to be examined, for example, the spinal column of a human being in the neck-head area, bending of the coil causes the butterfly and single flat coil to be misaligned. The resulting signals become distorted and do not reproduced faithfully the desired magnetic fields produced by the body under examination. The flat coils while satisfactory for some applications where the body under examination is planar and therefore equidistant from the stacked coils, in other instances, is not as satisfactory when the body under examination is curved and non-planar. As a result, the magnetic fields induced by the body and to be detected by the quadrature coil system are spaced different distances from different portions of the coil causing distortions in the signal.
The present inventors recognize a need for a detection quadrature surface detection coil for use with a non-planar body under examination and which overcomes the drawbacks of the stack coil system described above. One problem with producing a coil system for a curved body is that any bends in the coils out of the planar region tend to distort the coil and make the coil non-symmetrical. Further, it is extremely difficult to axially align the magnetic field responsive region of the butterfly coil to the single flat coil once the coils are bent. If the bends are slightly off center, i.e., non-symmetrical, then the two different coil systems, the flat coil and the butterfly coil, tend to respond in a manner producing a distorted unsatisfactory signal. To manufacture bent coils which are exactly physically aligned to respond to aligned magnetic fields is not generally practical. To obtain an optimum signal, the stack of coils should be contoured to the object being examined aligned, and matched.
In a magnetic resonance imaging examination system an apparatus according to the present invention comprises a surface coil magnetic field gradient detection construction comprising an outer conductor forming a first coil and a transverse inner conductor connected at its ends to the outer conductor at spaced points along the outer conductor for dividing the outer conductor into mirror image second coils. The construction includes means for tuning the first and second coils to a given radio frequency such that the first and second coils generate corresponding first and second signals manifesting orthogonal magnetic fields in the body under examination.
A feature of the invention includes signal isolation means coupled to the first and second coils for isolating the first and second signals. By way of example, such isolation means may include 180.degree. phase shift means coupled to the outer conductor at the junction with one end of the transverse inner conductor. This phase shift means places the transverse inner conductor at virtual ground so that the outer conductor corresponds to a single turn coil while the inner conductor with the outer conductor forms a pair of coils corresponding to a butterfly coil, both coils being aligned regardless their bent state because they comprise a common coil structure.